Six couples holding banners saying "Marriage For All Japan" walked into Tokyo District Court to file their cases against the government, with similar cases filed by three couples in Osaka, one couple in Nagoya and three couples in Sapporo.

Plaintiff Kenji Aiba, standing next to his partner Ken Kozumi, told reporters he would "fight this war together with sexual minorities all around Japan".

Aiba and Kozumi have held onto a marriage certificate they signed at their wedding party in 2013, anticipating Japan would emulate other advanced nations and legalise same-sex unions.

That day has yet to come, and legally they are just friends even though they've lived as a married couple for more than five years. So they decided to act rather than waiting.

"Right now we are both in good health and able to work, but what if either of us has an accident or becomes ill? We are not allowed to be each other's guarantors for medical treatment, or to be each other's heir," Kozumi, a 45-year-old office worker, said in a recent interview with his partner Aiba, 40.

"Progress in Japan has been too slow."

Ten Japanese municipalities have enacted "partnership" ordinances for same-sex couples to make it easier for them to rent apartments together, among other things, but they are not legally binding.

In a society where pressure for conformity is strong, many gay people hide their sexuality, fearing prejudice at home, school or work. The obstacles are even higher for transgender people in the highly gender-specific society. The Supreme Court last month upheld a law that effectively requires transgender people to be sterilised before they can have their gender changed on official documents.

The LGBTQ equal rights movement has lagged behind in Japan because people who are silently not conforming to conventional notions of sexuality have been so marginalised that the issue hasn't been considered a human rights problem, experts say.

"Many people don't even think of a possibility that their neighbours, colleagues or classmates may be sexual minorities," said Mizuho Fukushima, a lawyer-turned-lawmaker and an expert on gender and human rights issues. "And the pressure to follow a conservative family model, in which heterosexual couples are supposed to marry and have children, is still strong."

Prime Minister Shinzo Abe and his ultra-conservative supporters have campaigned to restore a paternalistic society based on heterosexual marriages.

The government has restarted moral education class at schools to teach children family values and good deeds.

"Whether to allow same-sex marriage is an issue that affects the foundation of how families should be in Japan, which requires an extremely careful examination," Abe said in a statement last year.

The ruling Liberal Democratic Party has repeatedly come under fire for making remarks deemed discriminating against LGBTQ people. In January, party veteran Katsuei Hirasawa said "a nation would collapse" if everyone became LGBTQ. Last year, another ruling lawmaker, Mio Sugita, was condemned after saying in a magazine that the government shouldn't use tax money for the rights of LGBTQ individuals because they are "not productive".